Re: [PATCH v9 4/8] KVM: Use gfn instead of hva for mmu_notifier_retry

Chao Peng
2022-11-11T08:32:25Z

On Thu, Nov 10, 2022 at 08:06:33PM +0000, Sean Christopherson wrote:
> On Tue, Oct 25, 2022, Chao Peng wrote:
> > @@ -715,15 +715,9 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
> > kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
> > }
> >
> > -void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> > - unsigned long end)
> > +static inline
>
> Don't tag static functions with "inline" unless they're in headers, in which case
> the inline is effectively required. In pretty much every scenario, the compiler
> can do a better job of optimizing inline vs. non-inline, i.e. odds are very good
> the compiler would inline this helper anyways, and if not, there would likely be
> a good reason not to inline it.

Yep, I know the rationale behind, I made a mistake.

>
> It'll be a moot point in this case (more below), but this would also reduce the
> line length and avoid the wrap.
>
> > void update_invalidate_range(struct kvm *kvm, gfn_t start,
> > + gfn_t end)
>
> I appreciate the effort to make this easier to read, but making such a big divergence
> from the kernel's preferred formatting is often counter-productive, e.g. I blinked a
> few times when first reading this code.
>
> Again, moot point this time (still below ;-) ), but for future reference, better
> options are to either let the line poke out or simply wrap early to get the
> bundling of parameters that you want, e.g.
>
> static inline void update_invalidate_range(struct kvm *kvm, gfn_t start, gfn_t end)
>
> or
>
> static inline void update_invalidate_range(struct kvm *kvm,
> gfn_t start, gfn_t end)

Fully agreed.

>
> > {
> > - /*
> > - * The count increase must become visible at unlock time as no
> > - * spte can be established without taking the mmu_lock and
> > - * count is also read inside the mmu_lock critical section.
> > - */
> > - kvm->mmu_invalidate_in_progress++;
> > if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> > kvm->mmu_invalidate_range_start = start;
> > kvm->mmu_invalidate_range_end = end;
> > @@ -744,6 +738,28 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> > }
> > }
> >
> > +static void mark_invalidate_in_progress(struct kvm *kvm, gfn_t start, gfn_t end)
>
> Splitting the helpers this way yields a weird API overall, e.g. it's possible
> (common, actually) to have an "end" without a "begin".
>
> Taking the range in the "end" is also dangerous/misleading/imbalanced, because _if_
> there are multiple ranges in a batch, each range would need to be unwound
> independently, e.g. the invocation of the "end" helper in
> kvm_mmu_notifier_invalidate_range_end() is flat out wrong, it just doesn't cause
> problems because KVM doesn't (currently) try to unwind regions (and probably never
> will, but that's beside the point).

I actually also don't feel good with existing code (taking range in the
"start" and "end") but didn't go further to find a better solution.

>
> Rather than shunt what is effectively the "begin" into a separate helper, provide
> three separate APIs, e.g. begin, range_add, end. That way, begin+end don't take a
> range and thus are symmetrical, always paired, and can't screw up unwinding since
> they don't have a range to unwind.

This looks much better to me.

>
> It'll require three calls in every case, but that's not the end of the world since
> none of these flows are super hot paths.
>
> > +{
> > + /*
> > + * The count increase must become visible at unlock time as no
> > + * spte can be established without taking the mmu_lock and
> > + * count is also read inside the mmu_lock critical section.
> > + */
> > + kvm->mmu_invalidate_in_progress++;
>
> This should invalidate (ha!) mmu_invalidate_range_{start,end}, and then WARN in
> mmu_invalidate_retry() if the range isn't valid. And the "add" helper should
> WARN if mmu_invalidate_in_progress == 0.
>
> > +}
> > +
> > +static bool kvm_mmu_handle_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
>
> "handle" is waaaay too generic. Just match kvm_unmap_gfn_range() and call it
> kvm_mmu_unmap_gfn_range(). This is a local function so it's unlikely to collide
> with arch code, now or in the future.

Agreed.

>
> > +{
> > + update_invalidate_range(kvm, range->start, range->end);
> > + return kvm_unmap_gfn_range(kvm, range);
> > +}
>
> Overall, this? Compile tested only...

Thanks!
Chao
>
> ---
> arch/x86/kvm/mmu/mmu.c | 8 +++++---
> include/linux/kvm_host.h | 33 +++++++++++++++++++++------------
> virt/kvm/kvm_main.c | 30 +++++++++++++++++++++---------
> 3 files changed, 47 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 93c389eaf471..d4b373e3e524 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4259,7 +4259,7 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,
> return true;
>
> return fault->slot &&
> - mmu_invalidate_retry_hva(vcpu->kvm, mmu_seq, fault->hva);
> + mmu_invalidate_retry_gfn(vcpu->kvm, mmu_seq, fault->gfn);
> }
>
> static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> @@ -6098,7 +6098,9 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
>
> write_lock(&kvm->mmu_lock);
>
> - kvm_mmu_invalidate_begin(kvm, gfn_start, gfn_end);
> + kvm_mmu_invalidate_begin(kvm);
> +
> + kvm_mmu_invalidate_range_add(kvm, gfn_start, gfn_end);
>
> flush = kvm_rmap_zap_gfn_range(kvm, gfn_start, gfn_end);
>
> @@ -6112,7 +6114,7 @@ void kvm_zap_gfn_range(struct kvm *kvm, gfn_t gfn_start, gfn_t gfn_end)
> kvm_flush_remote_tlbs_with_address(kvm, gfn_start,
> gfn_end - gfn_start);
>
> - kvm_mmu_invalidate_end(kvm, gfn_start, gfn_end);
> + kvm_mmu_invalidate_end(kvm);
>
> write_unlock(&kvm->mmu_lock);
> }
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index e6e66c5e56f2..29aa6d6827cc 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -778,8 +778,8 @@ struct kvm {
> struct mmu_notifier mmu_notifier;
> unsigned long mmu_invalidate_seq;
> long mmu_invalidate_in_progress;
> - unsigned long mmu_invalidate_range_start;
> - unsigned long mmu_invalidate_range_end;
> + gfn_t mmu_invalidate_range_start;
> + gfn_t mmu_invalidate_range_end;
> #endif
> struct list_head devices;
> u64 manual_dirty_log_protect;
> @@ -1378,10 +1378,9 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc);
> void *kvm_mmu_memory_cache_alloc(struct kvm_mmu_memory_cache *mc);
> #endif
>
> -void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> - unsigned long end);
> -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> - unsigned long end);
> +void kvm_mmu_invalidate_begin(struct kvm *kvm);
> +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end);
> +void kvm_mmu_invalidate_end(struct kvm *kvm);
>
> long kvm_arch_dev_ioctl(struct file *filp,
> unsigned int ioctl, unsigned long arg);
> @@ -1952,9 +1951,9 @@ static inline int mmu_invalidate_retry(struct kvm *kvm, unsigned long mmu_seq)
> return 0;
> }
>
> -static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
> +static inline int mmu_invalidate_retry_gfn(struct kvm *kvm,
> unsigned long mmu_seq,
> - unsigned long hva)
> + gfn_t gfn)
> {
> lockdep_assert_held(&kvm->mmu_lock);
> /*
> @@ -1963,10 +1962,20 @@ static inline int mmu_invalidate_retry_hva(struct kvm *kvm,
> * that might be being invalidated. Note that it may include some false
> * positives, due to shortcuts when handing concurrent invalidations.
> */
> - if (unlikely(kvm->mmu_invalidate_in_progress) &&
> - hva >= kvm->mmu_invalidate_range_start &&
> - hva < kvm->mmu_invalidate_range_end)
> - return 1;
> + if (unlikely(kvm->mmu_invalidate_in_progress)) {
> + /*
> + * Dropping mmu_lock after bumping mmu_invalidate_in_progress
> + * but before updating the range is a KVM bug.
> + */
> + if (WARN_ON_ONCE(kvm->mmu_invalidate_range_start == INVALID_GPA ||
> + kvm->mmu_invalidate_range_end == INVALID_GPA))
> + return 1;
> +
> + if (gfn >= kvm->mmu_invalidate_range_start &&
> + gfn < kvm->mmu_invalidate_range_end)
> + return 1;
> + }
> +
> if (kvm->mmu_invalidate_seq != mmu_seq)
> return 1;
> return 0;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 43bbe4fde078..e9e03b979f77 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -540,9 +540,7 @@ static void kvm_mmu_notifier_invalidate_range(struct mmu_notifier *mn,
>
> typedef bool (*hva_handler_t)(struct kvm *kvm, struct kvm_gfn_range *range);
>
> -typedef void (*on_lock_fn_t)(struct kvm *kvm, unsigned long start,
> - unsigned long end);
> -
> +typedef void (*on_lock_fn_t)(struct kvm *kvm);
> typedef void (*on_unlock_fn_t)(struct kvm *kvm);
>
> struct kvm_hva_range {
> @@ -628,7 +626,8 @@ static __always_inline int __kvm_handle_hva_range(struct kvm *kvm,
> locked = true;
> KVM_MMU_LOCK(kvm);
> if (!IS_KVM_NULL_FN(range->on_lock))
> - range->on_lock(kvm, range->start, range->end);
> + range->on_lock(kvm);
> +
> if (IS_KVM_NULL_FN(range->handler))
> break;
> }
> @@ -715,8 +714,7 @@ static void kvm_mmu_notifier_change_pte(struct mmu_notifier *mn,
> kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);
> }
>
> -void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> - unsigned long end)
> +void kvm_mmu_invalidate_begin(struct kvm *kvm)
> {
> /*
> * The count increase must become visible at unlock time as no
> @@ -724,6 +722,15 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> * count is also read inside the mmu_lock critical section.
> */
> kvm->mmu_invalidate_in_progress++;
> +
> + kvm->mmu_invalidate_range_start = INVALID_GPA;
> + kvm->mmu_invalidate_range_end = INVALID_GPA;
> +}
> +
> +void kvm_mmu_invalidate_range_add(struct kvm *kvm, gfn_t start, gfn_t end)
> +{
> + WARN_ON_ONCE(!kvm->mmu_invalidate_in_progress);
> +
> if (likely(kvm->mmu_invalidate_in_progress == 1)) {
> kvm->mmu_invalidate_range_start = start;
> kvm->mmu_invalidate_range_end = end;
> @@ -744,6 +751,12 @@ void kvm_mmu_invalidate_begin(struct kvm *kvm, unsigned long start,
> }
> }
>
> +static bool kvm_mmu_unmap_gfn_range(struct kvm *kvm, struct kvm_gfn_range *range)
> +{
> + kvm_mmu_invalidate_range_add(kvm, range->start, range->end);
> + return kvm_unmap_gfn_range(kvm, range);
> +}
> +
> static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> const struct mmu_notifier_range *range)
> {
> @@ -752,7 +765,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> .start = range->start,
> .end = range->end,
> .pte = __pte(0),
> - .handler = kvm_unmap_gfn_range,
> + .handler = kvm_mmu_unmap_gfn_range,
> .on_lock = kvm_mmu_invalidate_begin,
> .on_unlock = kvm_arch_guest_memory_reclaimed,
> .flush_on_ret = true,
> @@ -791,8 +804,7 @@ static int kvm_mmu_notifier_invalidate_range_start(struct mmu_notifier *mn,
> return 0;
> }
>
> -void kvm_mmu_invalidate_end(struct kvm *kvm, unsigned long start,
> - unsigned long end)
> +void kvm_mmu_invalidate_end(struct kvm *kvm)
> {
> /*
> * This sequence increase will notify the kvm page fault that
>
> base-commit: d663b8a285986072428a6a145e5994bc275df994
> --
>